Capacitive touch control is established on the detection of capacitance variation at the touch position on a touch sensor. The touch sensor is made of conductor, and thus there is always an insulator formed over the touch sensor as a cover to protect the conductor.
FIG. 1 is a schematic diagram of a capacitive touch device 10 touched by a finger 16, in which the capacitive touch device 10 includes a touch sensor 12 and a cover 14 on the touch sensor 12. Since the touch sensor 12 is made of conductor, there will be a self capacitance Cs established between the touch sensor 12 and ground GND. When the finger 16 contacts the cover 14, since the cover 14 is made of insulator and the human body is a conductor itself and has a potential equal to the ground potential, a coupling capacitance Cp appears between the touch sensor 12 and the finger 16. In this case, the touch sensor 12 and the finger 16 may be regarded as the opposite electrodes of the coupling capacitance Cp, respectively, with the cover 14 as the dielectric of the coupling capacitor Cp. As a result, the coupling capacitance Cp is connected in parallel to the self capacitance Cs of the touch sensor 12, causing the overall self capacitance to increase. Therefore, the finger 16 can be detected by sensing variation in the self capacitance.
In addition to detection of variation in the self capacitance, conventional touch position detection may be alternatively accomplished by detecting variation in the mutual capacitance between two electrodes in a touch sensor. For example, as shown in FIG. 2, a touch sensor includes two sensor electrodes 18 and 20, between which a mutual capacitance is established by lines of the electric field between the sensor electrodes 18 and 20. When a finger 16 approaches the touch sensor, the lines of the electric field between the sensor electrodes 18 and 20 are attracted by the large grounded capacitance of the human body, causing the mutual capacitance between the sensor electrodes 18 and 20 to decrease. Therefore, the finger 16 can be detected by sensing variation in the mutual capacitance.
FIG. 3 is a schematic diagram of a two-dimensional capacitive touch panel with single sensor layer, in which the left drawing shows the layout 21 of the touch sensor in a top view, and the right drawing shows a cross-sectional view of a sensing units 22 along the line A-A in the layout 21. The single sensor layer includes a plurality of sensing units 22, each of which has two sensor electrodes 24 and 26 electrically disconnected with each other, and each of the sensor electrodes 24 and 26 has an area per unit length complementally varying along the X direction in FIG. 3. When a conductor 28, e.g. a finger, contacts the cover 14 on the sensing unit 22, the conductor 28 can be regarded as an electrode to establish two capacitors with the sensor electrodes 24 and 26 respectively, each having a capacitance
                              C          ∝                                    ɛ              ×              A                        d                          ,                            [                  Eq          ⁢                      -                    ⁢          1                ]            where A represents the area of the conductor 28 overlapping the sensor electrode 24 or 26, and d and ∈ represent the thickness and dielectric constant of the cover 14, respectively. As shown by the equation Eq-1, the capacitances established by the conductor 28 and the sensor electrodes 24 and 26 are proportional to the areas of the conductor 28 overlapping the sensor electrodes 24 and 26, respectively. When the conductor 28 touches at the position X1 on the sensing unit 22, the variations ΔC1 and ΔC2 in the self capacitances sensed from the sensor electrodes 24 and 26 by a detector 30 will represent the capacitances established between the conductor 28 and the electrodes 24 and 26, respectively, which are proportional to the areas of the conductor 28 overlapping the sensor electrode 24 or 26, respectively. In addition, since the sensor electrodes 24 and 26 have their areas per unit length complementally varying as they extend in the X direction, the variations ΔC1 and ΔC2 in the self capacitances sensed from the sensor electrodes 24 and 26 are related to the position X1. Thus, by using the variations ΔC1 and ΔC2 in the self capacitances sensed from the sensor electrodes 24 and 26 and the length L of the sensor electrodes 24 and 26, the detector 30 can identify the conductor 28 at the position
                              X          ⁢                                          ⁢          1                =                              f            ⁡                          (                                                Δ                  ⁢                                                                          ⁢                  C                  ⁢                                                                          ⁢                  1                                ,                                  Δ                  ⁢                                                                          ⁢                  C                  ⁢                                                                          ⁢                  2                                ,                L                            )                                ≈                      L            ×                                                            Δ                  ⁢                                                                          ⁢                  C                  ⁢                                                                          ⁢                  2                                                                      Δ                    ⁢                                                                                  ⁢                    C                    ⁢                                                                                  ⁢                    1                                    +                                      Δ                    ⁢                                                                                  ⁢                    C                    ⁢                                                                                  ⁢                    2                                                              .                                                          [                  Eq          ⁢                      -                    ⁢          2                ]            To identify the position of the conductor 28 in the Y direction in FIG. 3, the detector 30 only has to identify which sensing unit 22 has its self capacitances changed. This is how a single sensor layer can accomplish two-dimensional position detection.